Ecology of Indonesian Papua Part Two. Andrew J. Marshall
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from about 3 to 120 million, with about 5–10 million being most likely. About 85% of all species are arthropods, and a majority of those are insects. On coral reefs, one estimate is that 93,000 species may have been described, but the total may be ten times higher (Reaka-Kudla 1995a,b). However, at the level of the largest groups of animals, known as phyla (such as Mollusca, Arthropoda, and Chordata) there are more phyla of multicellular animals (metazoa) in the oceans (about 29 out of 32) than on land (about 13) and in freshwater (about 16) (Rupert and Barnes 1994), and coral reefs probably have more phyla than any other ecosystem (about 26).
Relationships among Organisms
Coral reefs have very large numbers of species living together and interacting in a very complex web of relationships. We have already spoken of the mutualistic relationship between corals and the zooxanthellae they host. There are many other examples of coral reef species that have mutualistic relationships with other species, such as the Anemonefish (Amphiprion) that live among the tentacles of sea anemones; cleaner fish, such as Labroides dimidiatus, and shrimp, such as Periclimenes, that clean parasites off of fish; snapping shrimp (Alpheids) that excavate burrows which they and guardian prawn gobies (Gobiidae), occupy, and the Guard Crabs (Trapezia) that live in the branches of corals (Pocillopora) and defend them.
In another type of symbiosis, commensalism, one organism lives on another and benefits, while the host organism is neither helped nor hurt by the relationship. An example is the shrimp Periclimenes that live on sea anemones, starfish, nudibranchs (sea slugs) and other animals. A third type of symbiosis is parasitism, where one partner benefits at the expense of the other. Many small crustaceans and flatworms live on the skin of fishes, eating mucus and tissue off of the fish. Some crustaceans called isopods live attached on the outsides of fish, sucking tissue and blood. There are so many species of flatworms that are parasitic on snails in at least one stage of their life cycle that it is said that nearly every species of snail is parasitized by a species-specific parasitic flatworm. One parasitic flatworm, Plagioporus, lives in corals, Porites, at one stage in its life, causing the host polyp to expand and turn pink. The larger pink polyp stands out and is often eaten by butterflyfish (Chaetodontidae). This transfers the flatworm to its next host (Aeby 1991).
New relationships like these are being discovered all the time. For example, the snail Dendropoma maxima produces an uncoiled shell in a coral. The snail, called a vermatid because its shell resembles a worm tube, secretes mucus, which it drapes over the coral surface. The snail pulls the mucus in, dragging along with it additional mucus produced by coral, and eats it. The snail is thus parasitic on the coral, and stunts the growth of the surrounding coral (Fenner, under review a). Infectious diseases are similar to parasites, except that the agents that cause the diseases are generally microorganisms such as protozoa, bacteria, and viruses. Coral diseases have increased in number and severity in recent years, both on Caribbean and Indo-Pacific reefs. Diseases were detected at 10 of 45 (22%) sites during a recent survey at the Raja Ampat Islands (McKenna, Boli, and Allen 2002).
Nearly all animals on coral reefs are either predators or herbivores, the exceptions being those hosting endosymbiont algae such as corals. Predation may shape some communities, with apex predators dominating fish communities in pristine coral reefs such as the northwest Hawai’ian Islands. Predators may increase diversity by preying on the most abundant species. Two important predators of corals are the Crown-of-Thorns (COTS) starfish (Acanthaster planci) and snails of the genus Drupella. Crown-of-Thorns starfish came to the attention of scientists and the public in the mid-1960s when there were large outbreaks on the Great Barrier Reef in Australia, and nearly all corals were eaten and killed on some reefs in the midsection of the Great Barrier Reef. Outbreaks were reported on many reefs in the Indo-Pacific, with some of the worst around Okinawa, Japan (Moor 1989). Reports of outbreaks are much less frequent today, and reefs around Okinawa are now largely free of outbreaks. Outbreaks continue to occur on the Great Barrier Reef periodically. The first outbreak of COTS reported in Indonesia occurred in 1995, on the reefs of the Seribu Islands south of Papua (Tomascik et al.1997). In the Raja Ampats, only 3 of 45 (6.7%) sites had any COTS at all (McKenna, Boli, and Allen 2002). A few outbreaks of the snail Drupella have been reported in the Indo-Pacific (Moyer, Emerson, and Ross 1982), but they have not done as much damage as Crown-of-Thorns starfish. In the Raja Ampats study, Drupella were only observed at one site.
The Coral Triangle: The Peak of Diversity
The amazing diversity of species is perhaps the most notable aspect of the coral reefs of Papua Province. Papua is situated within the area that has been called the ‘‘Coral Triangle’’ (e.g., Allen 2002a; Wells 2002). This is the area of the highest diversity coral reefs in the world (Figure 5.2.4). It includes the Philippines, central and eastern Indonesia, and Papua New Guinea. There are more coral species in this area than in anywhere else in the world (Hughes, Bellwood, and Connolly 2002; Stehli and Wells 1971; Veron 1995), and the same is true of fish (Allen 2002a; Hughes, Bellwood, and Connolly 2002; Chapter 4.8), mollusks (Gosliner 2002; Wells 2002), and sponges (van Soest 1997). A recent expedition to the Solomon Islands found that the diversity of corals, fishes, and mollusks there is equally high, indicating that it too is part of the Coral Triangle (Green, Veron, and Allen 2004; Wilkinson 2004). Further, the number of mollusks in the Coral Sea area (Solomons to Great Barrier Reef to Vanuatu) is nearly as high as in the Coral Triangle (Wells 2002). In addition, recent work has found that diversity levels on outer barrier reefs of the northern Great Barrier Reef were marginally below that typical in the Coral Triangle (Fenner, unpublished). Coral diversity on mid-shelf reefs is even higher (Done 1982), so it is likely that the northern Great Barrier Reef has coral diversities within the range of diversities of the Coral Triangle.
Figure 5.2.4. Map of coral species diversity. Coral diversity decreases in all directions from the Coral Triangle (indicated in the darkest shade), which contains 581 coral species.
Source: Reproduced from Veron (2000) with kind permission from the author.
DIVERSITY GRADIENTS
The number of species decreases in all directions from the Coral Triangle (Figure 5.2.4). The diversity gradient to the north and south from the Coral Triangle is called the Latitudinal Diversity Gradient. This is perhaps best illustrated by the gradient in species diversity in southern Japan, with 342 species of coral in the small islands just north of Taiwan, decreasing almost in a straight line to zero species around Tokyo (Figure 5.2.5). A similar diversity gradient extends eastward from the Coral Triangle, with the number of species decreasing to the east in the Pacific until in the eastern Pacific there are a total of only about 33 species over a very large area. This gradient is called the Longitudinal Gradient. Both the Latitudinal and Longitudinal gradients have been documented in several groups of organisms. For instance, Indonesia has 90 species of Crinoids (feather stars), and going north, Palau has 30 and Guam has six. Going east, the Marshall Islands have 14 species and Hawai’i has none. Similarly, there are 536 species of sea slugs (opisthobranchs) known from the north coast of Papua New Guinea, 410 in Guam, 244 in Hawai’i, and 183 in Pacific Panama (Gosliner 1992). The diversity gradient for corals and fish shows a less steep gradient going west from the Coral Triangle in the Indian Ocean. Recent work in the Red Sea by Emre Turak, Lyndon Devantier, and J. E. N. Veron (reflected in the maps in Veron 2000) has doubled the number of species known there from 150 to 300, showing that the Indian Ocean Longitudinal Gradient is not as steep as previously thought.
Figure 5.2.5. Coral diversity in Japan decreases with increasing latitude (n 10, R2 0.96).
Source: Redrawn from Veron (1992).
A recent study (Karlson, Cornell, and Hughes 2004) compared coral species richness on a longitudinal gradient, comparing the number of species in Indonesia, Papua New Guinea, the Solomon Islands, American Samoa, and the Society Islands of French Polynesia. There were three sites in each country, with one of the sites in Indonesia being an island site to the west of the Vogelkop Peninsula of Papua. They separated transects on reef slopes, reef crests, and reef flats, and had an equal number of transects at each location and each reef zone. They found that on reef slopes, there